1. Field of the Invention
The invention relates to an arrangement for countering an undesired operation of directional overcurrent protection relays situated in parallel feeders of each phase of a multiphase AC power system, in particular--but not exclusively--a three-phase power system.
2. Description of the Related Art
Overcurrent protection systems are known in which (see FIG. 1) a multiphase (normally 3-phase) power source 10 feeds a correspondingly multiphase load 11 by way of respective source and load busbars 12, 13 and an arrangement of first feeders 20 and parallel-connected second feeders 30 linking these busbars. Three sets of relays 21/31, 22/32 and 23/33 are connected in the feeders 20/30. Each feeder 20 and 30 has a non-directional overcurrent protection relay 21 and 31, respectively, on the load side of the source busbars 12 and a directional overcurrent relay 22 and 32, together with a non-directional overcurrent relay 23 and 33, respectively, on the source side of the load busbars 13. These latter two sets of relays may be realized in each case as a single set of devices having two different characteristics (see later). As an alternative arrangement (FIG. 2), the feeders 20/30 may be connected to the secondaries 24 of respective transformers 25/35, the primaries 26/36 being fed from respective sets of non-directional relays 21/31.
To explain the mode of operation of overcurrent relays in general, reference will be made to FIG. 3, in which a three-phase feeder is shown in simplified form as a single line containing, in this case, three sections each having a non-directional relay 40/41/42 and an associated current transformer 50/51/52 and circuit-breaking device 60/61/62. The relays are arranged to operate at decreasing overcurrent settings from AC source 10 to fault location F and to have similarly decreasing response times, which in a typical system might involve a difference of around 0.4 s from one relay to the next. When a fault F occurs as shown, it is required that only the circuit breaker 62 in the section concerned be tripped, the remaining breakers continuing to provide a closed path to maintain supply at substations A, B and C which may be feeding loads 63 as shown. Thus, due to the lower fault current levels associated with the location of this fault (the impedance of the feeder is greater) and the above-mentioned so-called grading between the relays in terms of current and time-response, only relay 42 will operate in response to the fault current, leading to tripping of the circuit breaker 62. In like manner, were a fault to develop in the middle section, the fault current would be greater and would serve to operate the relay 41, and thereby trip the circuit breaker 61, and so on.
The relays may have any of three response characteristics, namely instantaneous-time (this is assumed not to be the case in FIG. 3), dependent-time and independent-time. The later two characteristics arc shown in FIG. 4, which is a graph of time "t" against current "I". The curve 70 represents a dependent-time characteristic and curve 71 an independent-time characteristic. Curve 70 provides a response time which varies according to the level of fault current seen by the relay, response being slower at lower current values than at higher current values. For curve 71, the response time is invariant when fault-current exceeds a certain value I.sub.1.
Directionality of a relay is achieved by the use of a reference quantity, normally system voltage, against which current is compared, the relative phase between these two quantities determining the "direction" of the current. The relay then only responds if that direction is the one to which the relay has been configured to respond. Thus, a directional relay has two inputs: voltage and current. The reference voltage is often referred to as the "polarizing voltage".
Referring now to FIGS. 5(a) and (b), these represent a simplification of FIGS. 1 and 2, respectively, inasmuch as the three-phase system is represented as a single line diagram containing two parallel-connected feeders and the associated directional and non-directional relays. If it is assumed that no fault exists on the AC system and that the system is working at full capacity, the feeders 20/30 will each pass half the total rated load current I.sub.L. However, the relays will be designed to have a minimum safe overcurrent setting in excess of the maximum load current I.sub.L on the assumption that one of the feeders might be out of service. In the event a fault develops on one of the feeders, that current setting may be exceeded for one or more relays, causing that or those relays to register a fault current level (i.e., to "pick up") and, following a time delay, to trip its associated circuit breaker (i.e., to "time out"). Of course, the "time out" only occurs if the fault is not cleared before expiry of the relay's time delay.
In the case of FIG. 5, it is assumed that a fault has developed on one feeder at the location F1 shown. The fault may be either phase-to-phase or phase-to-earth. In the absence of the fault, power flow was from source to load and therefore the directional relays 80, 81 were nominally insensitive to the normal load current which consequently flowed. However, with the appearance of the fault at F1, current flow changes so that, if the fault current is represented as I.sub.F1, a proportion of that current (e.g., I.sub.F1 /2) will flow into the fault branch via non-directional relay 82 and the remainder (I.sub.F1 /2) via non-directional relay 83 and directional relay 91. The actual value of fault current in each branch is dependent on fault position and will not necessarily split equally between the branches. Under these circumstances current flow in the relay 81 is in the right direction for it to operate. It will in fact operate provided that firstly, the fault current exceeds its fault-current threshold, and secondly, its set response time is shorter than the duration of the fault. Similar considerations hold for the transformer feeder arrangement of FIG. 5(b).
FIGS. 6(a) and 6(b) show two possible relay co-ordination diagrams for the various relays shown in FIG. 5. In the first case, the relays are configured as dependent-time relays having the characteristic shown as curve 70 in FIG. 4, but with the non-directional relays 84, 85 and 86 graded in terms of response time for a fault current I.sub.F2. The current-threshold settings of relays 83 and 85 are identical; (these settings are, incidentally, stored in non-volatile memory in the relay). It is the fact that relays 83 and 85 are set the same which causes the initial problem (which is resolved by the use of directional overcurrent relays), i.e., that for a fault at F1, relay 85 could operate prior to relay 83, effectively isolating the load, since relay 82 would also operate. The directional relays 80, 81 are arranged to operate (trip their associated circuit breaker) at a lower current level, as it is commonly perceived that their setting is not restricted by the value of load current. The situation is similar in FIG. 6(b), but with all the relay characteristics being of the independent-time type.
As mentioned briefly earlier, it is possible to combine the functions of relays 81 and 83 in one unit, and this may be either a microprocessor-based or numerical relay having two separate sets of overcurrent protection elements.
It is known that the use of a directional relay in each branch can serve as an effective method of isolating all feeder faults with minimal system disruption. Further to this, it has also been recognized as desirable to set the current threshold of the directional relays below normal rated load current, since this can assist with the co-ordination (grading) of the various relays present in the system, in the manner illustrated in FIG. 3. However, there has to date been no guidance offered as to how far below load current such relays may be set while maintaining reliability of operation. Indeed, it has even been suggested that directional protection is not responsive at all to load current in the non-operating direction, so that effectively no problem exists regarding the actual setting used. Other authorities imply that 50% of load current may be a suitable rule of thumb.